In active-matrix organic light emitting diode (AMOLED) display technologies, display brightness is in direct proportion to drive current of an OLED device. At the moment when the OLED device is turned on, a pixel circuit provides corresponding drive current for the OLED device to form a current path from power supply voltage ELVDD to a cathode ELVSS of the OLED. After the power supply voltage ELVDD is inputted from outside of an effective display area, the power supply voltage ELVDD is transferred to each pixel circuit through a wire within the effective display area. Because of certain resistance of the wire, the power supply voltage ELVDD may generate a direct-current voltage drop (generally referred to as IR drop) in the transfer process.
Existence of the IR drop leads to uneven distribution of the power supply voltage ELVDD within the effective display area. This is specifically because the actual power supply voltage of each pixel circuit is VDD_pixel=ELVDD−I*R, wherein I represents an electric current value of ELVDD signal network, and R represents an electric resistance of a wire from the pixel circuit to an input terminal of the power supply voltage ELVDD. The wires from each pixel circuit to the input terminal of the power supply voltage ELVDD have different lengths. Therefore, each wire has different electric resistances R, i.e., the IR drops are different. When a drive transistor has uneven saturation, each pixel circuit has different pixel currents, which causes nonuniform display. Furthermore, as display panels are increased in size, the problem of IR drop becomes increasingly severe, which leads to nonuniform panel display brightness.
Furthermore, drift of threshold voltage of the drive transistor in the pixel circuit also may lead to nonuniform panel display brightness, and a hysteresis effect of the drive transistor may cause short-term image sticking, thereby having a negative effect on the panel display quality.